This invention relates to isolation valves in hydraulic brake systems of railway vehicles and particularly to transit-type railway vehicles which employ spring-applied, hydraulic pressure released brake units.
Hydraulic brake systems employing spring-applied, pressure-released brake units are inherently safe from a braking standpoint, due to the fact that loss of hydraulic pressure for any reason will result in the brakes being automatically applied under spring force. Any loss of hydraulic fluid, however, presents a safety hazard which is unrelated to braking, due to its environmental impact. Hydraulic fluid which is spewed onto the track area is not only a pollutant, but also presents a fire hazard, especially in the vicinity of third rail electrical systems.
So-called isolation valves are typically employed in these hydraulic brake systems to prevent such loss of hydraulic fluid, in the event of a ruptured brake line. Generally, an isolation valve is located between the hydraulic pressure controller on each railway car and the brake units of the respective railway car truck. The isolation valve must sense a reduction in pressure in the brake circuit leading to the brake units of either truck and cut off the supply of hydraulic fluid to the truck having a leak in its brake circuit, while continuing to establish hydraulic pressure communication with the other truck brake units. In cutting off the supply of hydraulic fluid to the truck brake units having a leak in its brake circuit, without also cutting off the hydraulic fluid supply to the other truck brake units, only a single truck of the railway car experiences loss of brake control.
One known type of isolation valve relies on the differential pressure effective on opposite sides of a spool valve to shift the spool valve from a central position in which it is normally maintained by two springs, when a hydraulic leak occurs in either truck brake circuit. As long as the downstream pressure remains substantially equal in the separate truck braking circuits, the spring centers the spool valve, to establish hydraulic pressure communication between the controlled input pressure and each one of the respective truck brake circuits. In the event of leakage in either one of the truck brake circuits downstream of the isolation valve, the resultant low pressure therein creates the differential pressure to shift the spool valve in the appropriate direction from its normal central position and thereby cut off the supply of hydraulic fluid to the leaking brake circuit, while continuing to conduct hydraulic fluid to the other brake circuit.
If one of the centering springs breaks or binds for any reason, however, the spool will be forced to one side and will cut off the supply of hydraulic brake fluid to that particular brake circuit, even in the absence of any leakage whatsoever in that circuit. This will prevent the brakes from being released following a brake application, when an isolation valve of the type described is used in a hydraulic brake system having spring-applied, hydraulic pressure released brake units.